I do both LOL. The trail-a-bike is hooked to my bike and the trailer is hooked to the trail-a-bike. Last summer DS1 was 5 and DS2 was 3. This summer I will either have the baby in a seat on the back of my bike (in addition to the "train") or in the infant car seat inside the trailer with DS2. We have an in-step brand trailer and a Trek trail-a-bike.

I used to see a family do that for the morning commute with 2 kids. Then they graduated to a tandem + trail a bike.